In general, LCDs are divided into an in-plane switching (IPS)-LCD, a super in-plane switching (super IPS)-LCD, and a fringe field switching (FFS)-LCD according to modes of an active matrix driving electrode including pairs of electrodes. In the present invention, IPS-LCDs include the super IPS-LDS and the FFS-LCD.
FIG. 1 illustrates the basic structure of a conventional IPS-LCD. The IPS-LCD comprises a first polarizing plate 1, a second polarizing plate 2, and an IPS panel 3. An absorption axis 4 of the first polarizing plate 1 is orthogonal to an absorption axis 5 of the second polarizing plate 2, and is parallel with an optical axis 6 of a liquid crystal in the IPS panel 3.
Polarizing films in the first and second polarizing plates 1 and 2 are very thin and stretched films, thus being easily damaged by external physical and mechanical force. Accordingly, in order to protect the polarizing films of the first and second polarizing plates 1 and 2, a protective film is essentially formed on the internal surfaces of the first and second polarizing plates 1 and 2, on which the liquid crystal is formed.
In order to compensate for the light polarization of a polarizing plate to improve visibility and increase clearness of a screen, various phase retardation films are used. That is, in order to prevent light leakage and improve a contrast ratio, various phase retardation films or optical compensating films are used.
Accordingly, the polarizing plate includes various film layers, such as a polarizing film, a protective film for protecting the polarizing film, and a phase retardation film for improving optical characteristics of the polarizing plate. These various film layers increase the thickness of the polarizing plate.
This thick polarizing plate goes against the thin profile trend of the IPS-LCD. Thus, the improvement of the thick polarizing plate has been required.
In order to satisfy the above requirement, several techniques are proposed. According to one technique, the protective film is removed from the polarizing plate and the phase retardation film serves to protect the polarizing plate.
However, when the protective film is removed from the polarizing plate and the phase retardation film serves to protect the polarizing plate, the contrast ratio of the IPS-LCD is lowered. That is, the contrast ratio refers to a ratio of the luminance of the brightest portion to the luminance of the darkest portion. The higher a difference of luminances between the brightest portion and the darkest portion is, the higher the contrast ratio is. Accordingly, in order to assure the contrast ratio, it is necessary to prevent the light leakage at the darkest portion. When a phase retardation between the polarizing film and the phase retardation film is not properly controlled, the light leakage is severe according to viewable angles, thus causing a difficulty in assuring the contrast ratio.
Accordingly, an IPS-LCD comprising a polarizing plate, in which a phase retardation between a phase retardation film and a polarizing film is properly controlled, is required.
Particularly, the IPS-LCD has the worst contrast characteristic at a tilt angle of 75°. The improvement of the contrast characteristic of the IPS-LCD at a tilt angle of 75° means the improvement of the contrast characteristics of the IPS-LCD at all viewable angles. Thus, it is necessary to set the contrast characteristic of the IPS-LCD at tilt angle of 75° to more than a sufficient value.
For the above reason, a protective film is provided on the inner surfaces of the first and second polarizing plates of the IPS-LCD. In this case, the contrast ratio of the IPS-LCD is 10:1˜45:1. Consequently, it is preferable that an LCD having a contrast ratio similar to the above range and a simple structure is developed.